The present invention relates to a method and apparatus for applying a coating layer onto a substrate, and in particular, to a method of preparing a surface on a die coater to improve the uniformity of the coating layer on the substrate.
The production of high quality articles, particularly photographic, photothermographic, and thermographic articles, consists of applying a thin film of a coating solution onto a continuously moving substrate or web. Thin films can be applied using a variety of techniques, including dip coating, forward and reverse roll coating, wire wound rod coating, blade coating, slot coating, slide coating, curtain coating, and extrusion coating. Coatings can be applied as a single layer or as two or more superimposed layers. Although it is usually most convenient for the substrate to be in the form of a continuous web, it may also be formed to a succession of discrete sheets.
Die coating is a process whereby a pressurized stream of the coating material is moved through an internal manifold and discharged from an exit slot to form a ribbon of the coating material. The uniformity of the coating layer depends on the uniformity and precision of the coating slot on the coating dies.
One current method of manufacturing a coating die is to fabricate the die parts, such as by grinding or lapping procedures, and measuring the parts by conventional means. Conventional measuring techniques include laying the die parts on a flat surface, such as a granite table, with the surface being measured perpendicular to gravity. If the part is magnetic, it may even be measured for flatness on the grinder magnetic chuck. Conventional practice teaches that by measuring the die coating parts using this procedure, the best possible coating uniformity can be attained.
FIG. 1 is a schematic illustration of a prior method for measuring a die block 60. The die block 60 is located on a table surface 62 of measuring table 64 so that the machined surface 66 to be measured is perpendicular to gravity G. Gravity will act on the die block 60 in a way to influence the actual measured shape of the machined surface 66. The force of gravity causes the die block 60 to conform to the table surface 62 at an interface 68. Consequently, any residual stress within the die block 60 is distorted. Residual stress refers to a stress system within a solid that is not dependent upon external forces, such as gravity or a retaining fixture. Additionally, non-uniformity of the table surface 62 may be transmitted through the die block 60. The measured total indicated run-out of the machined surface 66 will not reflect the actual total indicated run-out when the die block 60 is in the production state. Production state refers to the orientation of the die block when mounted in the die assembly or a fixture simulating the die assembly.
In an attempt to improve coating uniformity, various manual, mechanical, thermomechanical, piezomechanical, magnetostrictive, and motor driven actuators have been installed on coating dies to control the die slot. The actuators can be located to generate an individual displacement force locally across the width of the slot exit. Since at any point across the die width the local discharge rate from the slot exit depends on the local gap, the uniformity of the flow rate from the die can be controlled across the width. U.S. Pat. No. 5,587,184 discloses a coating die with a slot thickness control mechanism located away from the slot exit.
Control of the die slot is typically accomplished by measuring the thickness of the film or coating at various points across its width with a thickness gauge such as a beta-ray, x-ray, or light absorption gauge. With the information from such measurements, an operator can manually adjust a bolt-type actuator bearing against the coating die. Alternatively, a control system can signal the activation of actuators which bear against the coating die or which rotate bolts that bear against the coating die. The manual adjustment of the coating die flexing bolts by an operator requires skill and experience. It has been shown that the quality of the product extruded or coated can be improved by a closed loop control system to replace the manual operator adjustment.
The die slot is typically not set for optimum uniformity when initially assembled. The adjustment cycle is time consuming and typically results in significant waste of coating material and substrate. Moreover, the actuators are not truly independent, but interact. That is, an adjustment of one actuator can require an adjustment of adjacent actuators. Consequently, the cross-web mechanical resolution, coupled with the limitations discussed above, results in inadequate accuracy of the die slot.
The present invention is directed to a method of improving the slot uniformity, and thus the coating uniformity, by accounting for forces acting on the die blocks that deflect their shape. These forces especially include the residual stress in the die block and gravity. Gravity can mask the effect of residual stress in the measurement of the die blocks when conventional measuring techniques are used. When the die block is mounted in the die assembly, the residual stress causes the total indicated run-out of the slot to be larger than anticipated and the slot to be distorted. Consequently, coating uniformity is degraded.
In the method of the present invention, the die blocks are measured in a free state whereby the residual stress is apparent. Subsequent machining can then be adjusted to compensate for the distortion due to residual stress, and a desired total indicated run-out achieved. The present invention is applicable to a variety of die coaters, including slide coaters, curtain coaters, extrusion coaters and slot coaters.
In a first embodiment, a surface of the die block is machined. The die block is then positioned on a measuring surface in a free state so that the machined surface being measured is substantially vertical. The vertical orientation of the die block substantially removes the effect of gravity on any residual stress during measuring. The steps of machining, positioning, and measuring the die block are repeated until the desired total indicated run-out is achieved. In an alternate embodiment, the positioning step may include interposing at least two point supports between the die block and the measuring surface.
In a second embodiment, the die block is machined and then positioned on a measuring surface so that the machined surface is in a reference state. Reference state refers to an orientation that simulates the production orientation of the die block during coating. The machined surface is measured and the steps of machining, positioning and measuring are repeated until a desired total indicated run-out is achieved.
In a third embodiment, the die block is machined and then positioned in a fixture that produces a simulated production state. The fixture includes fasteners corresponding to the fasteners used to assemble the die blocks for coating. The fasteners on the fixture generate forces that simulate the forces on the die block encountered in the die assembly during coating. The die block mounted in the simulated production state is measured for total indicated run-out. Machining, positioning, and measuring steps are repeated until a desired total indicated run-out is achieved.
The step of measuring is preferably performed using a non-contact measuring system, such as a laser interferometer. The surface being measured may be a slot wall or an alignment surface. In some embodiments, the die blocks have two or more surfaces prepared in accordance to the present invention.
The present method also includes assembling the die blocks prepared in accordance with the present invention, providing a flow of coating liquid to a manifold fluidly coupled to the die slot, and applying the coating liquid to a substrate.